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Chemistry > Chapter 21 > Flashcards

Chapter 21 Flashcards

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1
Q

What is a buffer solution?

A

A system which minimises pH changes when small amounts of acid or base is added. It contains two components when removing added alkali or acid:

  1. Weak acid (HA removed added alkali)
  2. The conjugate base (A- removes added acid).
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2
Q

What happens when one component is used up in the buffer?

A

It loses its buffer ability towards added acid or alkali.

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3
Q

Describe the preparation from a weak acid and its salt?

A

Makes a weak acid with a solution which contains the salt of the weak acid e.g. ethanoic acid and sodium ethanoate. The acid partially dissociates and the amount of ethanoate ions is very small.
CH3COOH (Comp 1) –> H+ + CH3COO-
Salts of acids are ionic compounds (conjugate base). When added to water the salt fully dissolves.
This is the conjugate base component.
CH3COONa + aq –> CH3COO- (Comp 2) + Na+

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4
Q

Describe the preparation by partial neutralisation?

A

Add an aqueous alkali like NaOH to an excess of a weak acid. The acid will partially neutralise resulting in a solution containing unreacted HA and A- salt produced.
CH3COOH + NaOH –> CH3COO-Na+ + H2O

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5
Q

Describe the ethanoic acid equilibrium?

A

Equilibrium lies towards the ethanoic acid. When CH3COO- ions are added to CH3COOH, it shifts further to the left reducing the H+ concentration leaving a 2 component solution. These act as reservoirs which can act independently to remove added acid or alkali.

CH3COOH (1) –> H+ + CH3COO- (2)

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6
Q

Describe the steps when the conjugate base removes acid on addition of an acid?

A
  1. The [H+] increases.
  2. H+ ions react with a conjugate base A- .
  3. Equilibrium position shifts to the left removing H+ ions.
    HA
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7
Q

Describe the steps when a weak acid removes alkali on an addition of an alkali?

A
  1. [OH-] increases.
  2. The small concentration of H + ions react with OH-
    H+ +OH- –> H2O
  3. HA dissociates due to this, the position shifts to the right to restore H+ ions.
    HA –> H+ + A-
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8
Q

What is the relationship between [H+][A-] in a buffer solution?

A

[H+] and [A-] are NOT equal

[HA] and [A-] are high

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9
Q

What happened to the buffer if [HA] = [A-]?

A

The pH of the buffer is the same as the Pka value of HA. The operating pH is typically over about two pH units centred at the pH of the Pka value. The ratio of the weak acid and conjugate base can be adjusted to fine-tune the pH of the buffer solution.

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10
Q

How do you workout [H+] in a buffer solution?

A

[H+] = ka x [HA] /[A-]

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11
Q

What is [HA] /[A-]

A

The acid: salt ratio

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12
Q

What does the acid salt ratio show?

A

Expresses that the pH depends on:
- Ka value of a weak acid.
- Ratio of HA : A-
both factors under control.

The ratio then provides the fine tuning of the buffers pH. If the ratio is two large or too small then they’ll be insufficient HA and A-.

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13
Q

What does the value of Ka provide?

A

The course tuning of the buffers pH.
You should assume the acid ratio is 1:1 when picking an appropriate weak acid. Ka value should be close to the desired H+ is possible.

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14
Q

What is the most effective buffer?

A

[HA] = [A-]

Therefore ka = [H+] and pKa = pH

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15
Q

What must the pH of blood to be between?

A

7.35 - 7.45

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16
Q

How is the pH of blood controlled?

A

By a number of buffers with the carbonic acid – hydrocarbonate (H2CO3/ HCO3-) buffer system.

17
Q

What happens if the pH falls below 7.35?

A

People develop acidosis which causes fatigue, shortness of breath, shock and death.

18
Q

What happens if the pH rises above 7.45?

A

People develop alkalosis which causes muscle spasms, lightheadedness and nausea.

19
Q

What is the equation for the blood buffer system?

A

H2CO3 –> H+ + HCO3-

20
Q

What happens when there is a drop in pH?

A

It means the hydrogen ion concentration has increased, the position of equilibrium shifts to the left to reduce the hydrogen concentration i.e. hydrogen carbonate reacts with excess H + ions

21
Q

What happens when there is a increase in pH?

A

It means the hydroxide concentration has increased is react with H+ to produce water. The position of equilibrium shifts to the right to restore H+ ions. Carbonic acid dissociates.

22
Q

How is pH monitored during a titration?

A

Using a pH meter with the data collected used to plot a titration curve. PH plotted against volume of acid/base added. A pH meter consist of an electrode which is dipped into the solution.

23
Q

What is the point of plotting a titration curve?

A

The purpose is to find the equivalent point (the point at which the two solutions have exactly reacted together). It’s determined by this stoichometry of the reaction and the concentration is used. The equivalent point is the centre of the vertical section of the pH titration curve.

24
Q

What is the titration experiment?

A
  1. Using a bulb pipette add a measured volume of acid to a conical flask.
  2. Place the pH meter electrode in the flask.
  3. Add the aqueous base to the burette and add acid to the conical flask 1 cm³ at a time.
  4. After each additions swirl the content, record the pH and the total volume of base added.
  5. Repeat 3 and 4 until the pH starts to turn more rapidly, then add drop wise for each reading until it changes less rapidly.
  6. Now add 1 cm³ of the base again until an excess has been added and the pH has been basic with little change for several additions.
25
Q

Describe the shape of the titration curve?

A

When the base is first added, the acid is in excess and the pH increases very slightly.
As the vertical section is approach to the pH start to increase more quickly as the acid is used more quickly.
The pH increases rapidly during addition of a small volume of base producing the vertical section. And a few drops are needed for the whole vertical section. After the vertical section the pH increases very slightly as the base is now in excess.

26
Q

What happens to the vertical section of a strong acid and strong base?

A

It’s centred at PH=7

27
Q

What happens to the vertical section of a weak acid and strong base?

A

It’s centred at PH>7

28
Q

What happens to the vertical section of a strong acid and weak base?

A

It’s centred at PH<7

29
Q

What happens to the vertical section of a weak acid and a weak base?

A

There is no vertical section

30
Q

What does the acid base indicator help us with?

A

Finding the equivalent point of a titration. The colour change could coincide with the vertical section of the equivalence point.

31
Q

What is an acid base indicator?

A

A weak acid HA which has a distant colour difference from the conjugate base A-.

32
Q

Explain methyl orange as an acid-base indicator?

A
  • weak acid HA is red (high H+ conc shifts it to the left)
  • when alkali is added OH- reacts with H+
  • more weak acid dissociates shifting the position to the right
  • endpoint [HA] = [A-] which produces an orange colour
  • as more alkali is added the equilibrium shifts to the right.
  • there is an excess of A- and the indicator is now yellow (conjugate base is yellow)
33
Q

Describe the relationship between [HA] and [A-] at the end point?

A

[HA] = [A-]

[H+] = Ka or pKa = pH

34
Q

How is an indicator chosen?

A

It’s chosen when the indicator has a colour change which coincides with the vertical section of the curve.

35
Q

What is the equivalence point?

A

The point in a titration when the volume of one solution has reacted exactly with the volume of the second solution.

36
Q

What indicator is suitable for a weak acid and a weak base?

A

There is no indicator suitable as there is no vertical section.

Chemistry (23 decks)

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